Stomach pain is one of the most bothersome phentermine side effects, albeit far from common. This inconvenient side effect can occur for a variety of reasons, but common causes of phentermine stomach pain include hunger, constipation and the medication itself. Phentermine abuse (i.e. taking it for longer or at higher doses than prescribed) may also produce serious conditions that initially present as abdominal pain [1,2].
Phentermine stomach pain can arise for a variety of reasons.
Potential causes of abdominal discomfort range from mild to severe, from easily managed at home to requiring emergency medical attention. As a result, it is important to remain mindful of this potentially-serious side effect.
Some of the most common reasons for phentermine stomach upset include:
Call your doctor if phentermine stomach pain is ongoing or occurs alongside other worrisome symptoms, especially if you take phentermine & topiramate together (e.g. Qsymia) or abuse phentermine.
Go to an ER if abdominal pain is severe, worsens, comes-on suddenly or occurs alongside chest pain.
Hunger is a common cause of stomach pain among stimulant users .
Phentermine helps patients lose weight by suppressing appetite and boosting energy. However, in some users – particularly new users – the appetite suppression proves too powerful and they forget to eat.
Phentermine stomach pain from hunger may seem trivial, but frequent, unintentional fasts can have a detrimental effect on your body, mood and nutritional status.
Major dietary changes can also cause phentermine upset stomach.
For example, eating more fiber (in the form of whole grains, fruit, vegetables and legumes) is undeniably beneficial in the long-run, but it may take your body and gut a few days or weeks to adjust to a new, healthier diet .
Adopting more radical diets, such a keto or intermittent fasting, may also entail a transition period that contributes to stomach pain.
Dehydration is another common cause of abdominal pain .
Stomach cells are highly sensitive to fluid status, so growing even moderately dehydrated can contribute to phentermine stomach pain. People who sweat a lot from exercise or hot weather, use diuretics (water pills) or have been recently ill with vomiting or diarrhea are at an increased risk of dehydration .
Constipation also leads to bloating, gas and phentermine stomach pain.
When the CNS is stimulated, either naturally or by phentermine, the body diverts resources away from the gastrointestinal (GI) tract to favor more critical “fight or flight” functions like the heart, lungs and skeletal muscles.
As a result, digestion slows while heart rate, respiration and reflexes increase. Unfortunately, prolonged CNS stimulation can cause constipation and/or delayed gastric emptying, both of which may contribute to phentermine stomach pain.
Phentermine stomach upset may also stem from the fact that the pills’ main ingredient is a CNS stimulant. Due to the widespread physical, hormonal and emotional changes associated with CNS activation, this medication can cause general GI distress.
Contact a doctor if your abdominal pain lasts for several hours or is accompanied by other worrisome symptoms including, but not limited to :
Stomach pain is one of the most serious phentermine and topiramate side effects. This reaction may indicate a severe condition called metabolic acidosis, which is a higher risk for patients taking this drug combination.
Contact your doctor right away if you experience severe abdominal pain or vomiting while taking phentermine and topiramate (or Qsymia). Other symptoms of metabolic acidosis include confusion, rapid breathing or pulse, abnormal heart rhythm, fatigue, weakness and shortness of breath .
Your phentermine stomach pain may still stem from something as simple as hunger or dehydration, (as discussed above), but a medical professional is the only person who can truly and accurately evaluate the severity of your symptoms.
Stomach pain is not listed as withdrawal symptom from Topamax (topiramate), but one week after stopping phentermine and topiramate, withdrawal stomach pain is sometimes reported.
Gastrointestinal problems like nausea, vomiting and stomach pain are common signs of withdrawal in general. These symptoms tend to arise as a result of neurotransmitter level variations after stopping phentermine.
If your phentermine and topiramate withdrawal stomach pain lasts for more than a couple hours, proves severe, worsens or occurs alongside other symptoms, contact your doctor as soon as possible.
The best way to relieve phentermine stomach pain is to find the cause of the discomfort and then address the root of the problem.
Try these simple, at-home remedies to alleviate minor phentermine stomach pain:
Given the powerful appetite suppressing qualities of phentermine, some users (especially new users) forget to eat. If you have phentermine stomach pain and a few hours have passed since your last meal or snack, grab a quick bite to eat.
Aim for real food snacks that contain a combination protein and slow-digesting carbohydrates, and total out at about 100-200 calories.
Prevent dehydration by drinking water or another calorie-free, caffeine-free beverage throughout the day. Urine should stay pale yellow and you should not feel thirsty.
As an added bonus, taking frequent sips of water is also one of the best ways to combat dry mouth: the most common side effect of phentermine diet pills.
Plus, including plenty of fluids (and fiber) in your daily diet helps prevent and treat constipation - another common cause of phentermine stomach pain.
If you suspect that constipation or slowed gastric motility may be behind your phentermine stomach pain, try going for a leisurely walk around the block.
Physical activity increases circulation throughout the body, including in the gastrointestinal tract. More blood flowing through the vessels surrounding the stomach and intestines promotes faster passage of food, which can help alleviate phentermine stomach pain related to constipation or slowed gastric emptying .
However, evidence remains mixed as to the effect of intense physical activity (such as running or an aerobic class) on GI symptoms worse, so steer clear of heart-pumping workouts until your phentermine stomach pain resolves – especially if they are not part of your usual routine .
If your phentermine upset stomach is not going away, or seems to be getting worse, call your doctor or go to a clinic right away. Any side effect that persists, worsens or interferes with your daily life should be evaluated by a medical professional.
It is especially important to contact a doctor ASAP if you experience abdominal pain alongside other worrisome symptoms or while taking phentermine and topiramate together.
Learn more about phentermine!
3. Robb, A. S. (2006, November 14). Recognizing and Managing Stimulant Complications.
4. Mayo Clinic Staff. (2018, November 16). How to add more fiber to your diet.
5. Shah, S. I., Aurangzeb, Khan, I., Bhatti, A. M., & Khan, A. A. (2004). Dehydration related abdominal pain (DRAP). [Abstract]. Journal of the College of Physicians and Surgeons – Pakistan, 14(1), 14-17. doi:01.2004/JCPSP.1417
6. Mayo Clinic Staff. (2018, February 15). Dehydration.
7. Department of Health & Human Services. (2012, March 31). Abdominal pain in adults.
8. Memorial Sloan Kettering Cancer Center. (2018, April 6). Phentermine and Topiramate.
9. WebMD Medical Reference. (2018, June 25). Exercise for Constipation Relief: Which Exercises to Do (M. W. Smith, MD, Ed.).
10. GI Society: Canadian Society of Intestinal Research. (2012). Physical Activity and GI Health.